The reaction of deoxy-sickle cell hemoglobin with hydroxyurea

In addition to its capacity to increase fetal hemoglobin levels, other mechanisms are implicated in hydroxyurea's ability to provide beneficial effects to patients with sickle cell disease. We hypothesize that the reaction of hemoglobin with hydroxyurea may play a role. It is shown that hydroxyurea reacts with deoxy-sickle cell hemoglobin (Hb) to form methemoglobin (metHb) and nitrosyl hemoglobin (HbNO). The products of the reaction as well as the kinetics are followed by absorption spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. Analysis of the kinetics shows that the reaction can be approximated by a pseudo-first order rate constant of 3.7x10(-4) (1/(s.M)) for the disappearance of deoxy-sickle cell hemoglobin. Further analysis shows that HbNO is formed at an observed average rate of 5.25x10(-5) (1/s), three to four times slower than the rate of formation of metHb. EPR spectroscopy is used to show that the formation of HbNO involves the specific transfer of NO from the NHOH group of hydroxyurea. The potential importance of this reaction is discussed in the context of metHb and HbNO being able to increase the delay time for sickle cell hemoglobin polymerization and HbNO's vasodilating capabilities through conversion to S-nitrosohemoglobin.